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这题主要还是按照题目意思去模拟就行了，但是js处理最大数字有点麻烦:

/**
 * @param {string} str
 * @return {number}
 */
var strToInt = function(str) {
   let res = 0,i = 0;
   while(str[i] === ' ' && i<str.length) {
       i++;
   }
   let flag = 0;
   if(str[i] === '-') {
       flag = 1;
   }
   if(str[i] === '+' || str[i] === '-') {
       i ++;
   } 
   let result = 0;
   while(i< str.length && str[i] >= '0' && str[i] <= '9') {
       res = str[i].charCodeAt() - '0'.charCodeAt();
       if(result >= Math.pow(2,31)) {
           return flag ? -Math.pow(2,31) : Math.pow(2,31)-1;
       }
       result = result * 10 + res;
       i ++;
   }
    if(result >= Math.pow(2,31)) {
        return flag ? -Math.pow(2,31) : Math.pow(2,31)-1;
    } else {
        return flag ? -result : result;
    }
};